Manholes are utilized for access purposes to underground conduits of utility lines and sewers. In the case of underground sanitary sewage infrastructure systems the manholes are usually installed with the entry-exit on a road surface. A concrete manhole conduit, or pipe, branches from the sewer and ultimately terminates at the road surface level where it is topped with a cast iron manhole unit and sealed from vehicles and pedestrians by a removable heavy cast iron manhole cover. The existing manhole units are approximately 7 inches in height and approximately 36 inches in diameter.
Since the manhole unit rests on a concrete manhole pipe that is connected to the underground sewage pipe system the manhole's absolute elevation remains fixed over time. However, in many cases the surface elevation will vary (up or down) over seasons and time. This is particularly prevalent in geographic locales that have a wide variance in climatic conditions from season to season and particularly where there is freezing and thawing conditions. As a result of ground shifts and heaving the manhole cover may dip below the road grade or protrude above and repairs associated with such elevation disparities are very costly for municipalities.
Even without adverse shifting of the road base, streets are periodically resurfaced for various reasons by the addition of paving material which may add one or two inches to the thickness of the existing surface. Driving over sunken or protruding manhole units results in vehicle damage as well as annoyance for the motorist. Motorists sometimes attempt to swerve to avoid protruding or sunken manhole units and such swerving can pose a safety threat to other motorists, cyclists, pedestrians or whomever may happen to be in the immediate vicinity. Cyclists not only face the same annoyances experienced by motorists but also face a greater threat to their physical safety.
Manhole units require considerable ongoing repairs if they are to remain unobtrusive on the driving surface. Repairs are not only expensive and time consuming but also disruptive to traffic flows. The repairs made do not fix the problem but only make temporary mends so long as the earth continues to heave and settle with resultant persistent elevational changes.
Numerous proposals have been made to overcome the foregoing problems by providing manhole units which can be adjustably varied in height and by way of example some of these proposals are illustrated in the following U.S. Pats.:
______________________________________ 4,273,467 issued June 16, 1981 4,149,816 issued April 17, 1979 3,858,998 issued January 7, 1975 3,773,428 issued November 20, 1973 3,392,640 issued July 19, 1968 2,930,295 issued March 29, 1960 1,076,836 issued October 21, 1913 4,925,337 issued May 15, 1990 4,281,944 issued August 4, 1981 4,197,031 issued April 8, 1980 3,611,889 issued October 12, 1971 4,075,796 issued February 28, 1978 1,165,044 issued December 21, 1915 5,054,956 issued October 8, 1991 ______________________________________
While the foregoing do provide for adjustability of the manhole cover support so as to bring the upper surface flush with the road surface they are not particularly practical because of the adjustment mechanisms being exposed in what might be considered a hostile environment. While some of the designs in theory should accomplish the desired results, their practicalities are not always favourable because of water, salt, sand, dust and the like environmental hostilities which can readily contact the exposed adjusting mechanism. Other proposed designs are complicated and thus expensive to manufacture.